The present invention relates to a method and apparatus for filtering an interlaced input digital video signal containing chrominance information.
Digital television signals include both a luminance signal usually referred to as a signal Y and Chrominance information in the form of two colour difference signals usually referred to as signals CB and CR. Such a television signal may be encoded in what is referred to as the 4:2:2 format so as to reduce the chrominance information to one half of that used for the luminance information in the horizontal direction. A signal in the 4:2:2 format may be converted to another format known as the 4:2:0 format in which the vertical chrominance information is further reduced by a factor of 2. The encoding of digital television signal parameters is effected at sampling frequencies determined by the format of the signal. A conversion from a 4:2:2 format to a 4:2:0 format is known as down-sampling and a conversion from a 4:2:0 format to a 4:2:2 format is known as up-sampling. When a down-sampling or an up-sampling conversion is performed, degradation of the chrominance information and hence in the picture quality may occur. Multiple conversions between the 4:2:2 format and the 4:2:0 format can result in successive degradation of the chrominance information.
A conventional filter used in a down-sampling conversion consists of several delay circuits, each introducing a delay of one line, coupled to a finite impulse response (FIR) filter. To avoid temporal blurring of the colour, the filter operates on each field of the picture independently but this reduces the bandwidth available for the chrominance information to at best one quarter of the original. In order to preserve bandwidth, the down conversion filter tends to be quite long (of the order of 6 or 7 taps) and as a result colour transitions can become visible, especially as such a filter may have large negative coefficients to achieve a satisfactory frequency response.
The present invention aims to provide for improved filtering of a digital video signal for a down-sampling conversion.
According to the present invention there is provided a method of filtering an interlaced digital video signal containing fields of chrominance information, the method comprising: applying the input video signal to a delay circuit to derive samples of the input signal representing spatially separated elements from each chrominance field of the signal; comparing the magnitudes of the samples relative to one another to identify frequencies which fall within a plurality of predetermined ranges; applying the input signal to an adaptive filter having a plurality of frequency response functions corresponding respectively to the predetermined frequency ranges; and, selecting a frequency response function in dependence upon the frequency range identified from the input signal samples.
Further according to the present invention there is provided filter apparatus for filtering an input interlaced digital video signal containing fields of chrominance information, the apparatus comprising: a delay circuit to receive the input video signal and to derive samples representing spatially separated elements from each chrominance field of the input signal; a comparator to compare the magnitudes of the samples relative to one another to identify frequencies within a plurality of predetermined frequency ranges; an adaptive filter having a plurality of frequency response functions corresponding respectively to the predetermined frequency ranges; and, a selector to select a frequency response function in dependence upon the frequency range identified by the comparator.
The invention has the advantage that the frequency response of the filter can be selected to present a flat response to low frequencies and a low-pass response to high frequencies so as to allow alias components to be removed from the signal. Furthermore the response functions can be made to contain no discontinuities between the response functions so that the filter does not introduce significant distortion of its own.